Ok, got it. The lightbox.js
file has this if block:
if(Prototype.Browser.Gecko) { overlay.setStyle({ backgroundImage: 'url('+this.options.overlay.presetImage+')', backgroundRepeat: 'repeat', height: this.pageDimensions.height + 'px' height: 700 + 'px' });
So basically, I just replaced the line where it sets height to height: 700 + 'px', it's weird that FF thinks the pages has over 1900px of height, because the page properties says it has only 1200. A firefox bug maybe?